Photosensitive elements obtained using photosensitive resin compositions with supports and protective films are widely used in the field of printed wiring board fabrication as resist materials for etching, plating and the like.
The two processes of tenting and plating are used for fabrication of printed wiring boards. Tenting is a method in which copper through holes for interlayer connection are protected with a resist and an electrical circuit is formed by etching and resist stripping, while plating is a method in which copper is deposited in the through holes by electroplating and protected by solder plating, and an electrical circuit is formed by resist stripping and etching.
Since tenting does not entail the steps of degreasing, substrate cleaning, acid washing, activation, etc. or involve prolonged contact of the resist with strongly acidic or strongly basic aqueous solutions, as is typical of plating, it is industrially useful in allowing avoidance of unwanted troubles that can occur with fabrication of printed wiring boards, and for simplifying the process.
When forming electrical circuits by such tenting methods, the following properties are required for the photosensitive resin composition:
(i) The film strength must be sufficient to withstand the spray pressure of the developing solution or cleaning water,
(ii) the resist must have a shape sufficient to obtain the desired line width for etching, and
(iii) fine strips must be produced by the stripping.
Stripping is usually accomplished with an automatic stripping machine, but when the strips are large they become tangled on the roll of the stripper, thereby not only significantly impairing the operation efficiency but also occasionally readhering to the clean substrate, and therefore fine strips are preferred. From the standpoint of line precision, the resist shape is preferably such that the resist is perpendicular without pitting in the resist wall at the interface between the resist and the copper surface. A trapezoidal resist shape will hinder achievement of high resolution, while an inverted trapezoidal shape will result in a relatively small contact area with the copper surface, thereby lowering adhesion of the resist during etching.
Particularly in the case of an alkali developing-type photosensitive resin composition, the size of the strips, the resist shape and the tent film strength will be determined by the composition and molecular weight of the binder polymer used in the photosensitive resin composition, and the strips will tend to be smaller with a lower molecular weight of the binder polymer and greater hydrophilicity of the composition. On the other hand, the resist shape will tend to be less suitable with a lower molecular weight of the binder polymer and greater hydrophilicity of the composition, with pitting appearing in the resist side walls. The tent film strength will tend to be lower with a lower molecular weight of the binder polymer and greater hydrophilicity of the composition, with breaks tending to occur in the tent film by the spray pressure of the developing solution or cleaning water.
The prior art has therefore not yet provided a photosensitive resin composition which gives a satisfactory resist shape and high tent film strength and resulting in fine strips. Consequently, even when a photosensitive resin composition with a satisfactory resist shape and high tent film strength is selected, large strips tend to result and become tangled onto the roll, and such strips must be removed from the roll several times each day.